Orinoco Womble (tidy bag and all) French is closer to Spanish than it is to English, linguistically, so I'd go with Spanish. I'm listening to the English audiobook and thinking I should have tried either French or Spanish.
Kumari de Silva I agree with the other readers that Spanish is closer to the original French, and if it is your first language you will probably enjoy it more. . .but if you find you like it I would recommend reading it second in English. It would probably be a really interesting exercise and would undoubtably stretch your vocabulary. I learned a lot of new words reading this book